Effect VirtualFileSystem

Run Effect programs against an in-memory filesystem without touching the host disk. Use it for isolated tests, build previews, or tools that need reproducible filesystem state.

Start with the memory filesystem

npm install @effect-vfs/memory effect@4.0.0-rc.114

import { MemoryFileSystem } from "@effect-vfs/memory"
import { Effect, FileSystem } from "effect"
 
const program = Effect.gen(function*() {
  const fs = yield* FileSystem.FileSystem
  yield* fs.writeFileString("/settings.json", JSON.stringify({ mode: "preview" }))
  return yield* fs.readFileString("/settings.json")
})
 
const settings = await Effect.runPromise(
  program.pipe(Effect.provide(MemoryFileSystem.layer))
)
 
console.log(settings)

The program continues to use Effect's standard filesystem interface. Only the provided layer changes, so production can use a host filesystem while tests and tools use disposable in-memory state.

Choose a package

• @effect-vfs/memory adapts the virtual filesystem to Effect's standard FileSystem.FileSystem service. This is the usual starting point.
• @effect-vfs/core provides byte-preserving paths, explicit callers and permissions, shared volumes, fixtures, quotas, watches, overlays, and portable encoded snapshots.
• @effect-vfs/persistence saves named snapshots to SQLite and restores them as fresh volumes in a later process.

All three packages are ESM-only and currently target effect@4.0.0-rc.114.